Surface-mount assembly in which electronic devices are mounted on electrically conductive pads on the surface of printed circuit boards, has assumed increasing importance to equipment manufacturers. In contrast to the through-hole technology in which the leads of electronic devices, such as DIP's or connectors, are inserted and soldered within plated-holes in the board, surface-mount assembly offers increased circuit density and lower production costs. Additionally, the surface mounting of devices provides improved high frequency circuit operation and increased reliability.
The "DIN" two-piece electrical connector configuration, comprised of a plug and mating socket is widely used. In designing for such connectors, where one of the sections is mounted on the edge of a printed circuit board, the alignment of the respective plug and socket bodies must be within .+-.0.004 inch. When only one connector section is mounted on the printed circuit board edge, the board is allowed to "float" in the card rack card guides in which it is disposed, to permit the board connector half to mate with the remaining connector half mounted on a backplane. However, when two or more connectors are mounted on the board edge, even though the board is permitted to "float", the location of the connector sections on the board with respect to each other, must be kept within the .+-.0.004 inch tolerance limit. Obviously, the same criterion must be met by the mating sections on the backplane if the connector sections are to be joined to each other.
In the case of the through-hole "DIN" connectors, the pins of the connector section to be mounted on the edge of a printed circuit board are inserted into corresponding metal-plated holes in the board and are soldered in place. With surface-mount connectors, the pins of the connector do not enter holes in the board, but instead, contact respective electrically conductive pads on the board surface. Each pin extremity of the surface-mount connector is bent to form a "foot" which rests upon a corresponding pad on the board. The pin is then reflow soldered to the pad.
As to the positional tolerance mentioned hereinbefore, the pin locations of the through-hole connector, the corresponding hole locations in the board, and the drill locations on both the board and connector are readily specified and held within the desired tolerance by precision manufacturing equipment. It should be noted that the maximum pin dimensions of the through-hole connector are less than the diameter of the hole dimensions of the board. Theoretically, this could result in a misalignment of mating connector sections. In practice however, it has been found that the effect of many pins in a common array located both on and off center as to the board holes in random fashion, tends to be compensatory such that the body of the connector becomes aligned within the required .+-.0.004 inch. As to the presence of a plurality of connector sections on a board edge, all holes for all connectors are drilled with the same numerically controlled drilling machine, which has an accuracy of .+-.0.0002 inch, which then insures a like positional tolerance from one connector section to another.
When employing surface-mount connectors which do not have pins protruding through plated through holes in the board, the aforementioned combined effect of the pins in aligning the body of the connector is absent. Accordingly, when a plurality of surface-mount connector sections are attached to the board in the recommended mounting configuration, utilizing standard hardware, the possible misalignment of the connector sections with respect to each other is likely to greatly exceed the allowable .+-.0.004 inch.
What is desired is a means for insuring the positions of a plurality of surface-mount connector sections relative to each other in their attachment to an edge of a printed circuit board. Such positioning is essential to successful engagement with respective connector mating sections on a backplane. The present invention provides an alignment fixture assembly for quickly and accurately accomplishing the foregoing.